I. Field of the Invention
This invention relates to the casting of metals, particularly metal alloys, and their treatment to make them suitable to form metal products such as sheet and plate articles.
II. Background Art
Metal alloys, and particularly aluminum alloys, are often cast from molten form to produce ingots or billets that are subsequently subjected to rolling, hot working, or the like, to produce sheet or plate articles used for the manufacture of numerous products. Ingots are frequently produced by direct chill (DC) casting, but there are equivalent casting methods, such as electromagnetic casting (e.g. as typified by U.S. Pat. Nos. 3,985,179 and 4,004,631, both to Goodrich et al.), that are also employed. The following discussion relates primarily to DC casting, but the same principles apply all such casting procedures that create the same or equivalent microstructural properties in the cast metal.
DC casting of metals (e.g. aluminum and aluminum alloys—referred to collectively in the following as aluminum) to produce ingots is typically carried out in a shallow, open-ended, axially vertical mold which is initially closed at its lower end by a downwardly movable platform (often referred to as a bottom block). The mold is surrounded by a cooling jacket through which a cooling fluid such as water is continuously circulated to provide external chilling of the mold wall. The molten aluminum (or other metal) is introduced into the upper end of the chilled mold and, as the molten metal solidifies in a region adjacent to the inner periphery of the mold, the platform is moved downwardly. With an effectively continuous movement of the platform and correspondingly continuous supply of molten aluminum to the mold, an ingot of desired length may be produced, limited only by the space available below the mold. Further details of DC casting may be obtained from U.S. Pat. No. 2,301,027 to Ennor (the disclosure of which is incorporated herein by reference), and other patents.
DC casting can also be carried out horizontally, i.e. with the mold oriented non-vertically, with some modification of equipment and, in such cases, the casting operation may be essentially continuous. In the following discussion, reference is made to vertical direct chill casting, but the same principles apply to horizontal DC casting.
The ingot emerging from the lower (output) end of the mold in vertical DC casting is externally solid but is still molten in its central core. In other words, the pool of molten metal within the mold extends downwardly into the central portion of the downwardly-moving ingot for some distance below the mold as a sump of molten metal. This sump has a progressively decreasing cross-section in the downward direction as the ingot solidifies inwardly from the outer surface until its core portion becomes completely solid. The portion of the cast metal product having a solid outer shell and a molten core is referred to herein as an embryonic ingot which becomes a cast ingot when fully solidified.
As an important feature of the direct chill casting process, a continuously-supplied coolant fluid, such as water, is brought into direct contact with the outer surface of the advancing embryonic ingot directly below the mold, thereby causing direct chilling of the surface metal. This direct chilling of the ingot surface serves both to maintain the peripheral portion of the ingot in solid state and to promote internal cooling and solidification of the ingot.
Conventionally, a single cooling zone is provided below the mold. Typically, the cooling action in this zone is effected by directing a substantially continuous flow of water uniformly along the periphery of the ingot immediately below the mold, the water being discharged, for example, from the lower end of the mold cooling jacket. In this procedure, the water impinges with considerable force or momentum onto the ingot surface at a substantial angle thereto and flows downwardly over the ingot surface with continuing but diminishing cooling effect until the ingot surface temperature approximates that of the water.
Typically, the coolant water, upon contacting the hot metal, first undergoes two boiling events. A film of predominately water vapor is formed directly under the liquid in the stagnant region of the jet and immediately adjacent to this, in the close regions above, to either side and below the jet, classical nucleate film boiling occurs. As the ingot cools, and the nucleation and mixing effect of the bubbles subsides, fluid flow and thermal boundary layer conditions change to forced convection down the bulk of the ingot until, eventually, the hydrodynamic conditions change to simple free falling film across the entire surface of the ingot in the lowermost extremities of the ingot.
Direct chill cast ingots produced in this way are generally subjected to hot and cold rolling steps, or other hot-working procedures, in order to produce articles such as sheet or plate of various thicknesses and widths. However, in most cases a homogenization procedure is normally required prior to rolling or other hot-working procedure in order to convert the metal to a more usable form and/or to improve the final properties of the rolled product. Homogenization is carried out to equilibrate microscopic concentration gradients. The homogenization step involves heating the cast ingot to an elevated temperature (generally a temperature above a transition temperature, e.g. a solvus temperature of the alloy, often 30 above 450° C. and typically (for many alloys) in the range of 500 to 630° C.) for a considerable period of time, e.g. a few hours and generally up to 30 hours.
The need for this homogenization step is a result of the microstructure deficiencies found in the cast product resulting from the early stages or final stages of solidification. On a microscopic level, the solidification of DC cast alloys are characterized by five events: (1) the nucleation of the primary phase (whose frequency may or may not be associated with the presence of a grain refiner); (2) the formation of a cellular, dendritic or combination of cellular and dendritic structures that define a grain; (3) the rejection of solute from the cellular/dendritic structure due to the prevailing non-equilibrium solidification conditions; (4) the movement of the rejected solute that is enhanced by the volume change of the solidifying primary phase; and (5) the concentration of rejected solute and its solidification at a terminal reaction temperature (e.g. eutectic).
The resulting structure of the metal is therefore quite complex and is characterized by compositional variances across not only the grain but also in the regions adjacent to the intermetallic phases where relatively soft and hard regions co-exist in the structure and, if not modified or transformed, will create final gauge property variances unacceptable to the final product.
Homogenization is a generic term generally used to describe a heat treatment designed to correct microscopic deficiencies in the distribution of solute elements and (concomitantly) modify the intermetallic structures present at the interfaces. Accepted results of a homogenization process include the following:
1. The elemental distribution within a grain becomes more uniform.
2. Any low melting point constituent particles (e.g. eutectics) that formed at the grain boundaries and triple points during casting are dissolved back into the grains.
3. Certain intermetallic particles (e.g. peritectics) undergo chemical and structural transformations.
4. Large intermetallic particles (e.g. peritectics) that form during casting may be fractured and rounded during heat-up.
5. Precipitates (such as may be used to subsequently developed to strengthen the material) are formed during heat-up are dissolved and later precipitated evenly across the grain after dissolution and redistribution as the ingot is once again cooled below the solvus and either held at a constant temperature and allowed to nucleate and grow, or cooled to room temperature and preheated to hot working temperatures.
In some cases, it is necessary to apply thermal treatments to ingots during the actual DC casting process to correct differential stress fields induced during the casting process. Those skilled in the art characterize alloys into those that either crack post-solidification or pre-solidification in response to these stresses.
Post-solidification cracks are caused by macroscopic stresses that develop during casting, which cause cracks to form in a trans-granular manner after solidification is complete. This is typically corrected by maintaining the ingot surface temperature (thus decreasing the temperature—hence strain—gradient in the ingot) at an elevated level during the casting process and by transferring conventionally cast ingots to a stress relieving furnace immediately after casting.
Pre-solidification cracks are also caused by macroscopic stresses that develop during casting. However, in this case, the macroscopic stresses formed during solidification are relieved by tearing or shearing the structure, inter-granularly, along low melting point eutectic networks (associated with solute rejection on solidification). It has been found that equalizing, from center to surface, the linear temperature gradient differential (i.e. the temperature derivative surface to center of the emerging ingot) can successfully mitigate such cracking.
These defects render the ingot unacceptable for many purposes. Various attempts have been made to overcome this problem by controlling the surface cooling rate of an ingot during casting. For instance, in alloys prone to post-solidification cracking, Zeigler, in U.S. Pat. No. 2,705,353, used a wiper to remove coolant from the surface of the ingot at a distance below the mold so that the internal heat of the ingot would reheat the cooled surface. The intention was to maintain the temperature of the surface at a level above about 300° F. (149° C.) and, preferably, within a typical annealing range of about 400 to 650° F. (204 to 344° C.) Zinniger, in U.S. Pat. No. 4,237,961, showed another direct chill casting system with a coolant wiping device in a form of an inflatable, elastomeric wiping collar. This served the same basic purpose as that described in the above Zeigler patent, with the surface temperature of the ingot being maintained at a level sufficient to relieve internal stresses. In the example of the Zinniger patent, the ingot surface is maintained at a temperature of approximately 500° F. (260° C.), which is again in the annealing range. The purpose of this procedure was to permit the casting of ingots of very large cross section by preventing the development of excessive thermal stresses within the ingot.
In pre-solidification crack prone alloys, Bryson, in U.S. Pat. No. 3,713,479, used two levels of water spray cooling of lesser intensity to decrease the cooling rate and have it extend a greater distance down the ingot as the ingot descends and, as a result of this work, demonstrated the capability to increase overall casting rates realized in the process.
Another design of direct chill casting device using a wiper for removing cooling water is shown in Ohatake et al. in Canadian Patent 2,095,085. With this design, primary and secondary water cooling jets are used, followed by a wiper to remove water, with the wiper being followed by a third cooling water jet.